The premise of the story is there is psychotic killer simply called
the joker
This anonymous killer came to be because people created him out of rumors, The rumor was that if you hated someone you'd call a specific number and the joker would appear to punish that person.
As the rumor grew the myth became a fact and now everyone is at risk the boogeyman created by themselves will end them.
Rumors becoming true is a huge part of the story dynamics, you can spread rumors yourself and they end up becoming true, for exmpl to get the best weapons in the game you need to start a rumor a specific shop is selling them then you hire a guy to start spreading the rumor and then it becomes true.
That, mixed with a dark storyline which has good voice acting in important scenes PLUS mythological deities AND spell combinations makes this game a superb rpg that makes most final fantasy games look childish.